Various types of inspection systems have been devised in the past which use a visual scanning technique for determining the location of a defect or anomaly in the surface of an object. Such a system typically employs a conventional television camera which receives reflected light from the object's surface which is then converted to a display. Pattern recognition techniques are employed to scan the displayed image in order to locate the defect.
Many types of defects are not readily visually perceivable and therefore cannot be detected with conventional visual scanning techniques. For example, poor molecular bonding in a defective weld may not be revealed at the surface of the weld. In the field of gaging, visual scanning techniques which rely on differences in grey scale level to distinguish features are ineffective in thoses cases where adjacent but different features are the same color and intensity.
It is known that the amount of light energy absorbed by a material is dependent upon the type of bonding within the molecules which make up the material and that light reflected from such material is shifted in energy or wavelength by an amount commensurate with the type and concentration of molecular bonding. Others have used this principle in the past to identify the constituent elements of a material. That is to say, others have used a technique to identify the constituent elements of a material in which a beam of light of a known frequency was directed onto the surface of the material and the shift in frequency of the reflected light from the material was measured in order to identify the constituent elements. It is also known that sensing sensitivity may be increased by tuning the energy of the incident light beam to that of the energy gap of the material being inspected. This latter technique is sometimes referred to as resonance Roman spectroscopy.